This invention relates to application of a coating to an article, and more particularly, to apparatus and an improved method for applying an aluminide coating simultaneously to the interior and exterior of an article.
The aluminizing process is well-known in the art. The process is used to apply an aluminide coating to substrates, such as nickel-base superalloy articles. Some of these articles require a coating on both external surfaces and on internal surfaces. For some articles, such as turbine airfoils, the internal surfaces are complex and accessible only through narrow, restricted passageways.
Improved turbine engines have resulted in the need for improved methods for coating the turbine airfoils. The internal surfaces of advanced turbine hardware are now subjected to such elevated temperatures that uncoated portions of such hardware are oxidized in service. As the oxide scale is formed in these narrow, serpentine passageways, a thickened scale grows from the surface, resulting in a narrowing of the passageway, which in turn reduces the effectiveness of the cooling air. The result is that the degradation process proceeds at an ever-increasing rate. This accelerated oxidation process can be stopped only by effectively coating the entire interior passageway of the part.
Several methods currently exist for coating the interior and the exterior surfaces of articles, such as turbine airfoils. Each has limitations which may add significantly to the cost and time required to process the articles, or which may result in a failure to adequately coat the internal surfaces.
One of the methods currently used to coat articles is a pack diffusion process. In this process, the surfaces of the articles to be coated are embedded in a source of finely divided particles including an aluminum source, a volatile halide to act as a chemical transfer medium for the aluminum, and a ceramic phase, such as alumina powder, to prevent the agglomeration of the metallic components, and heated to an elevated temperature, usually at least 1200.degree. F., all within a retort. Among the problems encountered is the accumulation of powdered particles in the narrow passages or openings of the parts prevents the flow of the aluminum gas through the narrow serpentine passageways. The result is a lack of a coating in the blocked passageways. Even in openings which are not blocked, it has been observed that the coating thickness tends to be thicker at or near the opening, and decreases rapidly with increasing distance from the opening. This nonuniform coating frequently produces unacceptable airfoils.
Another method used to coat articles is referred to as an over the pack process, such as described in Benden, U.S. Pat. No. 4,132,816 dated Jan. 2, 1979. This process is a simple modification of the pack process, in that the parts to be processed are suspended over the powdered material rather than embedded in the powdered material. In order to deposit the gaseous aluminum, produced by the elevated temperature decomposition of aluminum in the presence of the halide activator, onto the suspended parts, an inert gas is introduced into the retort and passed over the surfaces to be coated. This process is an improvement over pack processes since no powder mixture is in contact with the surfaces, and hence cannot block the openings to the narrow passageways. Although an improvement for coating internal passageways as compared to pack processing, the only control over the deposition rate as the reaction proceeds is control over the argon gas pressure and modification of the reaction temperature and time at temperature. However, such a system lacks control over the coating of the internal passageways.
In a modification of this method described in Benden, U.S. Pat. No. 4,148,275 dated Apr. 10, 1979, the retort has two chambers for effecting simultaneous coating of the exterior and the interior portions of the article. Each chamber has separate powder mixtures. The improvement allows better control of the coating of the internal passageways by controlling the flow rate of the carrier gas into the chamber which contains the aluminum source, the inert filler and the metal halide. This control is in addition to control available through variations in temperature and time.
Another method of applying a coating to articles having narrow, serpentine passageways, such as turbine airfoils, is chemical vapor deposition, also referred to as CVD. In CVD, the part to be coated is placed in a retort. An aluminum-bearing gas is generated at an elevated temperature in a reaction chamber outside of the retort, typically by reacting an aluminum source with an activator gas, such as a halide. The aluminum halide gas is then transported from the reaction chamber to the retort by an inert carrier gas. The aluminum halide gas then reacts at the surface of the article, depositing an aluminide coating. The resultant, halide-bearing gas is exhausted from the retort. Complex fixturing and redundant equipment for internal and external coating is required to assure that the appropriate coating thickness is deposited on the internal surfaces of the parts as well as the external surface of the parts. This makes an expensive process even more costly.
There therefore exists a need for improved apparatus and improved processing methods for applying, in a cost-effective manner, a coating simultaneously and in a controlled fashion to articles requiring coating on interior and exterior surfaces.